Support a customizable internal server error page in WSGIResource

[exarkun]

#4019 added a number of extra error handling behaviors to WSGIResource so that when an unexpected exception occurs, the request will be responded to with a 500 (if possible). The response has no body, though. It should be possible to specify the error page to use as a custom resource or resource factory. This might make sense as an argument to WSGIResource.__init__ or as an overrideable method on that class.